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Doctors have successfully treated many young SCID patients before with various types of bone-marrow transplants. What distinguishes these two babies, researchers reported in Science last week, is that they are the first to be treated, apparently successfully, entirely by gene therapy. Says Dr. Alain Fischer, who headed the gene team at Hopital Necker-Enfants Malades in Paris: "Preliminary evidence is of a faster and more complete immune reconstitution after gene therapy...
Typically, gene therapists use viruses to deliver replacement genes into a patient's body. In a slight variation on that strategy, Fischer and his colleagues removed some of the children's bone marrow, sorted out the cells they needed and infected them with the gene-bearing virus. Then the researchers injected the treated cells back into their young patients. Within two weeks, the children started showing signs of a healthy immune system...
...complicate matters, adults have stem cells too. Lurking in the microscopic nooks and crannies of the brain, bone marrow and other organs, these stem cells live in a state of perpetual readiness. Then when, say, the lining of the intestine becomes worn, the body signals the appropriate stem cells to start a process called differentiation, in which they divide and give rise to lots of mature, fully functioning intestinal cells...
These adult stem cells appear to be fairly restricted in what they can become. (Stem cells in the bone marrow usually give rise to different types of blood cells; stem cells in the muscles generally give rise to muscle.) Otherwise, Kafkaesque as it may seem, you could wake up one morning to find that your foot had turned itself into a liver. In any case, while there's no controversy over the use of adult stem cells, their potential benefit as a therapy seems limited...
Stem cells are also highly responsive to their surroundings. Researchers have taken adult stem cells from the brains of rats and put them in bone marrow and watched, in astonishment, as they spewed out blood cells. True, they did not form all the different blood-cell types, just a few. But until then no one had known that adult stem cells could adapt even that much to their environment...